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I have some bent bicycle rims and would like to straighten them. I drew a basic scheme of what I want to do, the idea is to make a concrete plane mold as seen in the design to exert pressure on the bent rims. The details are on the figure. Would you have some design and/or material suggestion? It's OK if you think it's useless I will appreciate the advice. Thanks in advance!

STRAIGHTEN RIM TOOL

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  • Do note that if a rim is merely "wobbly", and there are not any distinct bends in it, then the spokes, properly adjusted, should be able to pull it "true" (even though purists would object that the spoke tensions will not be even). Commented Mar 18, 2015 at 11:57

2 Answers 2

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This is not going to work.

You won't permanently change the shape of the rim by just smooshing it between two concrete discs, as in order to permanently bend metal you need to exceed the yield stress of the metal and plastically deform it. This means that to bend something to a shape, it has to be precisely bent further than its ultimate designed bend, then elastically bend back to the destined spot. Your concrete discs will not achieve this.

It is tricky enough to form a suitably round rim from a perfectly straight piece of freshly extruded, homogeneous aluminum. Once the rim has been bent, it will have been cold worked, and the mechanical properties will not be uniform along the entire rims length. Thus, trying to precisely bend it back into shape will be impossible as the yield strength and elastic behaviors will not be fully known throughout the entirety of the rim.

Trying to heat treat it won't save you either as that will cause the rim to get softer as it will undo the designed-in cold working of when the rim is created at the factory.

Additionally, this scheme, even if it was possible, would only adjust side-to-side trueness, and would do nothing to adjust vertical trueness. You'd also have to remove the hub and spokes, and at that point you might as well just get a fresh rim. In fact if your rim got bent while it was a wheel, many of your spokes are going to be wrecked anyway.

If a rim is significantly bent, it's only future destination is recycling.

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  • I see you also have cried in Strength of Materials class. But please, as an accepted and highly upvoted answer, do mention that spoke tension can do wonders; throwing away is only for cracked rims or rich eco-unfriendly fellows.
    – Vorac
    Commented Jul 21, 2020 at 16:31
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I'm not a metallurgist, but I would think that once the rim is bent enough that you're looking to straighten it, it's been weakened enough that I wouldn't trust it for riding on.

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